Additional Vegetative Compatibility Groups in Colletotrichum coccodes Subpopulations from Europe and Israel

Genetic Differentiation of Verticillium dahliae Populations Recovered from Symptomatic and Asymptomatic Hosts

Verticillium dahliae is a soilborne fungal pathogen affecting many economically important crops that can also infect weeds and rotational crops with no apparent disease symptoms. The main research goal was to test the hypothesis that V. dahliae populations recovered from asymptomatic rotational crops and weed species are evolutionarily and genetically distinct from symptomatic hosts. We collected V. dahliae isolates from symptomatic and asymptomatic hosts growing in fields with histories of Verticillium wilt of potato in Israel and Pennsylvania (United States), and used genotyping-by-sequencing to analyze the evolutionary history and genetic differentiation between populations of different hosts. A phylogeny inferred from 26,934 single-nucleotide polymorphisms (SNPs) in 126 V. dahliae isolates displayed a highly clonal structure correlated with vegetative compatibility groups, and isolates grouped in lineages 2A, 2B⁸²⁴, 4A, and 4B, with 77% of the isolates in lineage 4B. The lineages identified in this study were differentiated by host of origin; we found 2A, 2B⁸²⁴, and 4A only in symptomatic hosts but isolates from asymptomatic hosts (weeds, oat, and sorghum) grouped exclusively within lineage 4B, and were genetically indistinguishable from 4B isolates sampled from symptomatic hosts (potato, eggplant, and avocado). Using coalescent analysis of 158 SNPs of lineage 4B, we inferred a genealogy with clades that correlated with geographic origin. In contrast, isolates from asymptomatic and symptomatic hosts shared some of the same haplotypes and were not differentiated. We conclude that asymptomatic weeds and rotational hosts may be potential reservoirs for V. dahliae populations of lineage 4B, which are pathogenic to many cultivated hosts.

Assignment of Colletotrichum coccodes isolates into vegetative compatibility groups using infrared spectroscopy: a step towards practical application

FTIR spectroscopy may provide a specific, rapid, and inexpensive method for the successful classification of Colletotrichum coccodes isolates into vegetative compatibility groups.

Vegetative compatibility groups in Colletotrichum coccodes subpopulations from Australia and genetic links with subpopulations from Europe/Israel and North America

Vegetative compatibility of 94 isolates of Colletotrichum coccodes from Australia originating from potato, soil, and a weed (Solanum esuriale) was tested using nitrate-nonutilizing (nit) mutants. Isolates distributed to six vegetative compatibility groups (VCGs), five of them multimember (24.5, 23.4, 13.8, 12.8, and 7.5% distribution) and only one composed of two isolates (2.1%); 15.9% of them were not assigned to any of the VCGs. Aggressiveness of 51 isolates representing all six VCGs was tested by mature green tomato bioassay: isolates assigned to AUS-VCG-4 were the most aggressive and those in AUS-VCG-3 the least (P < 0.05). Isolates from warmer climates and lower latitudes were more aggressive (P < 0.05). In addition, we report for the first time complementations between isolates from Australia (AUS); North America (NA); and Israel, The Netherlands, Scotland, France, Germany (EU/I). Isolates assigned to AUS-VCG-4 anastomosed with isolates assigned to EU/I-VCG-7 and NA-VCG-5 (which also anastomosed with each other). Isolates assigned to EU/I-VCG-6 anastomosed with isolates assigned to NA-VCG-2 and isolates assigned to AUS-VCG-2 anastomosed with isolates assigned to EU/I-VCG-2. The linkage between subpopulations could result from the limited exchange of seed tubers among continents, or could be due to, for instance, gene flow, selection, or a limited number of polymorphic vegetative incompatibility genes.

Variability in morphology and aggressiveness among North American vegetative compatibility groups of Colletotrichum coccodes

North American isolates of Colletotrichum coccodes, representing six vegetative compatibility groups (NA-VCG), were used to study morphological and pathogenic variability. The objective was to determine if variability in conidial and microsclerotial size was related to pathogenicity. Significant differences were detected in length, width, and length/width ratios of conidia as well as in the length and width of microsclerotia among the NA-VCGs. The longest and widest conidia were produced by isolates belonging to NA-VCG1 and the largest microsclerotia were produced by isolates of NA-VCG2. Conidial and microsclerotial lengths and widths also were affected significantly by type of growth medium. There was no relationship between the size of conidia and the size of microsclerotia among the NA-VCGs studied. Conidial and microsclerotial size may affect inoculum potential and survival as isolates of NA-VCG2 have been demonstrated to occur more frequently than other NA-VCGs. Aggressiveness of 17 isolates of C. coccodes representing six NA-VCG's was studied on three potato cultivars using foliar and root inoculation methods. C. coccodes infection reduced tuber weight in all cultivars with both inoculation methods although tuber weight reductions were significantly higher following root inoculations than foliar inoculations. Pathogenic aggressiveness varied among NA-VCGs. Isolates belonging to NA-VCG2 and 3 were the least aggressive on potato foliage and isolates of NA-VCG1, 2, 3, 4, and 5 produced higher microsclerotial density on all three cultivars compared with isolates of NA-VCG6. Across inoculation methods, isolates of C. coccodes belonging to NA-VCG2 and 6 were the most aggressive based on reductions in tuber weight. Umatilla Russet was the most susceptible cultivar to C. coccodes compared to other cultivars regardless of inoculation method. These results demonstrate variability in morphology and pathogenic aggressiveness among the NA-VCGs of C. coccodes but these traits are not related.